Semiconductor light-emitting element and method of producing the same

ABSTRACT

There is provided a semiconductor light-emitting element and a method of producing the same including high density and high quality quantum dots emitting light at a wavelength of 1.3 μm. A semiconductor light-emitting element has a first GaAs layer, a second InAs thin film layer having the plurality of InAs quantum dots formed on the first GaAs layer, a third InGaAs layer formed on the second InAs thin film layer having the plurality of InAs quantum dots, and a fourth GaAs layer formed on the third InGaAs layer, wherein the As source is As 2 .

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light-emitting elementand a method of producing the same. More particularly, the presentinvention relates to a semiconductor laser and a method of producing thesame that can be used at the 1.3 μm wavelength region for opticalcommunication.

In the semiconductor light-emitting element using InAs quantum dots,high density is essential in order to improve the quality. The quantumdots have a trade-off relationship between the high density and a longwavelength.

Conventionally, high density cannot be realized at the 1.3 μm wavelengthregion for communication. Quantum dots that emit light at a wavelengthof 1.3 μm or more have a density of about 2×10¹⁰ cm⁻². Quantum dots thatemit light at the 1.27 μm wavelength region have a density of 8.7×10¹⁰cm⁻². See Japanese Unexamined Patent Application Publication No.2001-24284. A GaInAs layer formed on an InAs layer cannot contain anincreased content of In because of the occurrence of a transition or thelike.

In view of the above, an object of the present invention is to provide asemiconductor light-emitting element and a method of producing the sameincluding high density and high quality quantum dots emitting light at awavelength of 1.3 μm.

SUMMARY OF THE INVENTION

In order to achieve the object, the present invention utilizes thefollowing solving means:

(1) An arsenic source is changed from As₄ to As₂.

(2) A growth temperature and a growth speed are optimized.

(3) An InGaAs layer having a high In content is used.

(4) An InGaAs layer with a modified composition is used.

Preferably, a planar semiconductor light-emitting element is used toadjust the area for handling light, and to increase the number ofquantum dots.

A first aspect of the present invention is a semiconductorlight-emitting element, having:

a first GaAs layer,

a second InAs thin film layer having the plurality of InAs quantum dotsformed on the first GaAs layer,

a third InGaAs layer formed on the second InAs thin film layer havingthe plurality of InAs quantum dots, and

a fourth GaAs layer formed on the third InGaAs layer,

wherein the As source is As₂.

According to the first aspect of the present invention, the second InAsthin film layer having the plurality of InAs quantum dots formed on thefirst GaAs layer is formed at a growth temperature of not less than 540°C. and at a growth speed of not less than 0.006 ML/s.

According to the first aspect of the present invention, thesemiconductor light-emitting element has a light emission wavelengthwithin a range of 1.28 to 1.34 mm, a surface density of not less than6×10¹⁰ cm⁻² and a half width of not more than 40 meV.

A second aspect of the present invention is a method of producing asemiconductor light-emitting element, having the steps of:

forming a GaAs layer on a semiconductor substrate,

forming an InAs thin film layer having the plurality of InAs quantumdots on the GaAs layer,

forming an InGaAs layer on the InAs thin film layer having the pluralityof InAs quantum dots, and

forming another GaAs layer on the InGaAs layer,

wherein As source is As₂.

According to the second aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C.

According to the second aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C. but not more than theevaporating temperature of indium.

According to the second aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth speed of not less than 0.006 ML/s.

According to the second aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C. and at a growth speed of notless than 0.006 ML/s.

A third aspect of the present invention is a laminated semiconductorlight-emitting element, having the plurality of the semiconductorlight-emitting elements of the first aspect stacked vertically, whereinthe first layer is laminated on the fourth layer.

According to the third aspect of the present invention, an InGaAs layerhaving a high In content is formed at an interface between the InAsquantum dots and the InGaAs layer, and wherein the amount of Incontained in the InGaAs layer is gradually decreased in the directionaway from the interface.

A fourth aspect of the present invention is a method of producing asemiconductor light-emitting element, having the steps of:

forming a GaAs layer on a semiconductor substrate,

forming an InAs thin film layer having the plurality of InAs quantumdots on the GaAs layer,

forming an InGaAs layer on the InAs thin film layer having the pluralityof InAs quantum dots,

forming another GaAs layer on the InGaAs layer, and

repeating the steps so that a desired number of the semiconductorlight-emitting elements are disposed,

wherein As source is As₂.

According to the fourth aspect of the present invention, an InGaAs layerhaving a high In content is formed at an interface between the InAsquantum dots and the InGaAs layer, and wherein the amount of Incontained in the InGaAs layer is gradually decreased in the directionaway from the interface.

According to the fourth aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C.

According to the fourth aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C. but not more than theevaporating temperature of indium.

According to the fourth aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth speed of not less than 0.006 ML/s.

According to the fourth aspect of the present invention, the InAs thinfilm layer having the plurality of InAs quantum dots is produced at agrowth temperature of not less than 540° C. and at a growth speed of notless than 0.006 ML/s.

According to the fourth aspect of the present invention, thesemiconductor light-emitting element is a planar semiconductorlight-emitting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a semiconductor light-emittingelement;

FIG. 2 is a sectional view of an InAs quantum dot structure including aGaAs layer, an InAs thin film, InAs quantum dots and another GaAs layerin that order from the bottom;

FIG. 3 shows a production process of the InAs thin film, the InAsquantum dots, and the GaAs layer on the other GaAs layer shown in FIG.2;

FIG. 4 is a graph showing a relationship between a light emissionwavelength peak and a quantum dot surface density;

FIG. 5 is a sectional view of a semiconductor light-emitting elementaccording to the present invention having a GaAs layer, an InAs thinfilm, InAs quantum dots, an InGaAs layer and another GaAs layer;

FIG. 6 shows a production process of the InAs thin film, the InAsquantum dots, the InGaAs layer and the GaAs layer on the another GaAslayer shown in FIG. 5;

FIG. 7 is a sectional view of a semiconductor light-emitting elementaccording to the present invention having three-layered InAs quantumdots;

FIG. 8 is a graph showing a relationship between a composition of anInGaAs layer and a critical thickness;

FIG. 9 shows a relationship between lattice constants of our proposedgradient composition InGaAs layer and conventional InGaAs layer:

FIG. 10 is a graph showing a relationship between a composition of anInGaAs layer and a light emission wavelength peak;

FIG. 11 is a graph showing an emission spectra of quantum dots of InGaAslayers having different In contents with a surface density of 1.1×10¹¹cm⁻²;

FIG. 12 is a part of spectra data shown in FIG. 11;

FIG. 13 is a graph showing an emission spectra of three-layered InAsquantum dots with a surface density of 1.1×10¹¹ cm⁻²;

FIG. 14 is a part of spectra data shown in FIG. 13;

FIG. 15 is a graph showing temperature dependency of light-emittingintensity of the semiconductor light-emitting element according to thepresent invention;

FIG. 16 is a graph showing an increase in light-emitting intensity byproviding high density of the sole semiconductor light-emitting elementaccording to the present invention;

FIG. 17 is a graph showing a relationship between a growth rate and asurface density of the quantum dots according to the present invention;

FIG. 18 is a sectional view of an InAs quantum dot laser havingthree-layered quantum dot laser elements according to the presentinvention;

FIG. 19 shows properties of a semiconductor laser element havingfive-layered high density quantum dots; and

FIG. 20 shows properties of the semiconductor laser element havingthree-layered quantum dots according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detailreferring to drawings.

Embodiment 1

FIG. 1 is a schematic sectional view of a semiconductor light-emittingelement for emitting light. The semiconductor light-emitting element hasan n-type semiconductor, a p-type semiconductor, and a light-emittinglayer that contributes to light emission and disposed between the n-typeand p-type semiconductors. When a voltage is applied to the element fromabove and below, positive holes flow into a light-emitting layer fromthe p-type semiconductor, and electrons flow into the light-emittinglayer from the n-type semiconductor. The positive holes and electronsflowing into the light-emitting layer are reconnected, and light havinga wavelength corresponding to materials is emitted from a side, above orbelow of the semiconductor light-emitting element. In particular, when aplanar semiconductor light-emitting element that emits light from aboveand below (at a plane of different semiconductor layers) is used, areasof the positive holes and electrons that flow into the light-emittinglayer are adjusted, whereby the number of the quantum dots to beprovided can be adjusted. In other words, increasing the areas of thepositive holes and the electrons flowed thereinto can increase thenumber of the quantum dots. Examples of the materials include a III-VGroup compound semiconductor. The semiconductor light-emitting elementcomprising GaAs is used in an infrared region of 900 nm or less. On theother hand, the semiconductor light-emitting element comprising InAsquantum dots can be used in a communication wavelength region of about 1to 1.5 μm. At present, the InAs quantum dots are mainly produced usingAs₄ source.

However, according to the present invention, As₂ source is used. Arsenicmaterials As₄ and As₂ have different diffusion lengths. Specifically,As₂ has longer diffusion length than As₄. Accordingly, InAs quantum dotsusing As₂ are larger than InAs quantum dots using As₄. This gives riseto the difference in light-emitting properties and density. The InAsquantum dots on GaAs comprising As₂ have a trade-off relationshipbetween the high density and a long wavelength similar to theconventional quantum dots using As₄.

The trade-off relationship has been studied. FIG. 2 is a sectional viewof an InAs quantum dot structure comprising As₂. FIG. 3 shows aproduction process thereof. As shown in FIG. 3A, As₂ is provided on aGaAs layer formed on a semiconductor substrate (not shown), and thegrowth is interrupted for 60 seconds. As shown in FIG. 3B, In and As₂are provided to grow an InAs layer by 2.4 Molecular Layers (ML). Asshown in FIG. 3C, a GaAs layer is grown by feeding Ga an As at thegrowth temperature of not more than 540° C. The feeding of As₂ ismaintained during the interruption of the growth for 60 seconds. An InAsthin film layer that is grown at Stranski-Krastanow (S-K) mode isprovided between the GaAs under layer and the InAs quantum dots. TheInAs thin film layer is grown by about 1.8 ML.

Light emission properties and a surface density of the InAs quantum dotsdepend on the growth temperature of the InAs layer and a feed speed ofthe InAs layer. FIG. 4 shows a relationship between a light emissionwavelength peak and a quantum dot surface density. In FIG. 4, awavelength is represented by nm, and a dot density is represented bydot/cm⁻².

The density of the InAs quantum dots is measured using a scanningelectron microscope (SEM). Light emission wavelengths of the GaAs layergrown on the InAs quantum dots are measured using a photo luminescent(PL) method.

The light emission is 1.303 μm and a surface density of the quantum dotsis 0.8×10¹⁰ cm⁻² when the InAs layer is grown at the growth rate of0.006 ML/s and at the growth temperature of 540° C. The light emissionis 1.210 μm and a surface density of the quantum dots is 3.2×10¹⁰ cm⁻²when the InAs layer is grown at the growth rate of 0.03 ML/s and at thegrowth temperature of 540° C. The light emission is 1.201 μm and asurface density of the quantum dots is 7.7×10¹⁰ cm⁻² when the InAs layeris grown at the growth rate of 0.1 ML/s and at the growth temperature of540° C. The light emission is 1.192 μm and a surface density of thequantum dots is 1.1×10¹¹ cm⁻² when the InAs layer is grown at the growthrate of 0.1 ML/s and at the growth temperature of 520° C. Thus, when thegrowth temperature is decreased and the growth speed is increased, thediffusion length of an atom becomes short, and thus high density can beobtained. It is therefore contemplated that higher density can beprovided by feeding the material at lower temperature and at higherspeed. The growth conditions are optimized to provide high densityexceeding 1.1×10¹¹ cm⁻², which is better than the conventional one (seeJapanese Unexamined Patent Application Publication No. 2001-24284).

Embodiment 2

FIG. 5 is a sectional view of a structure according to the presentinvention. FIG. 6 shows a production process thereof. As shown in FIG.5, a GaAs layer 3 is grown on a semiconductor substrate (not shown), andan InAs thin film layer 4, InAs quantum dots 5 and an InGaAs layer 6 areformed thereon and planarized. Finally, another GaAs layer 2 is formedon top to provide a semiconductor light-emitting element 1.

As shown in FIG. 6A, As₂ is provided on a GaAs layer, and the growth isinterrupted for 60 seconds. As shown in FIG. 6B, In and As₂ are providedto produce InAs quantum dots 5. As shown in FIG. 6C, the growthtemperature is decreased by 50° C. during the interruption of the growthfor 30 seconds while feeding As₂. As shown in FIG. 6D, In, Ga and As₂are fed to grow an InGaAs layer 6. As shown in FIG. 6E, Ga and As₂ arefed to grow a GaAs layer 2. The higher the In content of the InGaAslayer 6 is, the more the strain between the InAs quantum dots 5 and theGaAs layer can be reduced, thereby improving the properties of thequantum dots. The best composition of the InGaAs layer 6 isIn_(0.5)Ga_(0.5)As having a moderate strain between the InAs quantumdots and the InGaAs layer. The InGaAs layer 6 typically has an Incontent of 20% or less. If the In content is high, a film thicknessexceeds a critical value and a misfit transfer occurs. A half width ofthe light emission becomes wide causing degradation of the lightemission properties. According to the present invention, the thicknessof the InGaAs layer is decreased to avoid the misfit transfer due to thecritical thickness.

Thickness and Composition of InGaAs Layer

The critical thickness and the composition of the InGaAs layer aredetermined. The InGaAs layer having the high In content on the InAsquantum dots can effectively lengthen the wavelength. However, an areahaving no quantum dots (i.e., calculation area) produces a strain, whichadversely affects the light-emitting properties. Next, the criticalthicknesses of the InAs thin film layer having no quantum dots and theInGaAs layer (see FIG. 5) are calculated. FIG. 8 shows the results. InFIG. 8, the ordinate represents a critical thickness per InGaAs layer innm, and the abscissa represents a suffix “x” in the In_(x)Ga_(1-x)Aslayer. The Matthews method is used for calculation. FIG. 7 shows thethree-layered InAs quantum dots structure suitable for providing thehigh density in this case.

The layered structure is in the following order from the bottom:

the GaAs layer 3,

the InAs thin film layer 4,

2.4 ML of the InAs quantum dots 5,

the InGaAs layer 6 for burying the InAs quantum dots 5 and planarizing,

26 nm of the GaAs layer 2 (3),

the InAs thin film layer 4,

2.4 ML of the InAs quantum dots 5,

the InGaAs layer 6 for burying the InAs quantum dots 5 and planarizing,

26 nm of the GaAs layer 2 (3),

the InAs thin film layer 4,

2.4 ML of the InAs quantum dots 5,

the InGaAs layer 6 for burying the InAs quantum dots 5 and planarizing,and

26 nm of the GaAs layer 2.

As the InGaAs layer has the high In content, the critical thicknessthereof becomes thin. For example, when In_(0.25)Ga_(0.75)As is used forthe one-layer structure shown in FIG. 5, the critical thickness is 9 nm.In contrast, the same is used for the three-layered structure shown inFIG. 7, whereby the critical thickness is as thin as 2.9 nm. In otherwords, the three layers shown in FIG. 7 correspond to the one layershown in FIG. 5, i.e., one of the three layers shown in FIG. 7 has aboutone-third of the thickness or less of the one layer shown in FIG. 5,such that the three layers should be thin for lamination. In such astructure, when the InGaAs layer has a high In content, the criticalthickness does not degrade the light emission properties.

Composition Gradient

In the present invention, a composition gradient is applied. The InGaAslayer having the high In content is grown only at the interface betweenthe InAs quantum dots and the InGaAs layer to lengthen the wavelength,and the In content is decreased in the InGaAs layer in the directionaway from the interface. Thus, the strain at the interface between theInAs quantum dots and the InGaAs layer is significantly reduced, thetotal In content is decreased, and the wavelength can be lengthenedwithout the misfit transfer. As shown FIG. 9A, in the conventionalsemiconductor light-emitting element with InGaAs layer, the strain isapplied at the interface between InAs and InGaAs and at the interfacebetween InGaAs and GaAs. However, as shown in FIG. 9B, when thecomposition gradient InGaAs is used, the strain at the interface betweenInAs and InGaAs and at the interface between InGaAs and GaAs can bereduced.

Using As₂ described above, InAs quantum dots having a light emissionwavelength peak of 1.192 μm and a surface density of 1.1×10^(11 cm) ⁻²,and InAs quantum dots having a light emission wavelength peak of 1.201μm and a surface density of 7.7×10¹¹ cm⁻², InAs quantum dots having alight emission wavelength peak of 1.210 μm and a surface density of3.2×10^(11 cm) ⁻² are produced. The InGaAs layer had a thickness of 3.0nm. The InGaAs layer at the interface of InAs quantum dots had thecomposition of In_(0.1)Ga_(0.93)As, In_(0.12)Ga_(0.88)As andIn_(0.3)Ga_(0.7)As, respectively. The InGaAs layer at the GaAs interfacehad the composition of In_(0.7)Ga_(0.93)As, In_(0.12)Ga_(0.88)As andIn_(0.13)Ga_(0.87)As, respectively. FIG. 10 shows a relationship betweena composition of an InGaAs layer and a light emission wavelength peak.The light emission wavelength peaks shift to the longer wavelength. Inthe case of In_(0.1)Ga_(0.9)As, the wavelength is lengthened 56 nm. Inthe case of In_(0.25)Ga_(0.75)As, the wavelength is lengthened 112 nm.In the case of In_(0.3)Ga_(0.7)As, the wavelength is lengthened 133 nm.

As shown in FIGS. 11 and 12, the InAs quantum dots having a surfacedensity of 1.1×10¹¹ cm⁻² lengthen the wavelength to 1.308 μm and 1.325μm. The half widths are also improved from 38.0 mV to 22.3 meV and 29.7meV. The significant decrease in the half widths can be achieved bydecreasing the level of the dots caused by the large strain between theInAs quantum dots and the InGaAs layer, and promoting the light emissionof the original quantum dots. There is provided the same advantage evenif the composition gradient is a step-like gradient or a gradualgradient. When the composition gradient is increased, the In content ofthe InGaAs layer at the interface of InAs can be increased, therebyproducing good quality crystal. Ultimately, In_(0.5)Ga_(0.5)As can beprovided. In this case, the InAs quantum dots have a density of as highas 1.1×10¹¹ cm⁻², and can emit light at a wavelength of 1.3 μm.

The concept of the composition gradient can be applied to all typicalsemiconductors having high strain. When certain crystal having a certainlattice constant is used to produce different crystal having a differentlattice constant, the composition gradient is used, whereby an elementhaving good crystal can be produced. Examples of a combination ofmaterials include Si and SiGe, GaN and AlInN, InN and GaAIN, InAs andInGaAsP, and InAs and AlInAsP. As a matter of course, the same can beapplied to III-V Group compound semiconductors and II-VI Group compoundsemiconductors where crystal can be grown.

The three-layered structure is produced using InAs quantum dots having asurface density of 1.1×10¹¹ cm⁻² to determine the light emissionproperties by repeating the production process shown in FIG. 6 threetimes. Since As₂ having long diffusion length is used, the GaAs layer onthe InGaAs layer is not easily planarized. Accordingly, the growth speedof GaAs should be as low as 1.0 μm/hour or less. The growth temperatureof the upper (second and third) layers is preferably the same or 5° C.higher as/than that of the lower (first and fourth) layers. FIGS. 13 and14 shows the light emission properties. The light emission wavelengthpeak is 1.309 μm and the half width is 23.2 meV. The narrow half widthcan be obtained by providing good quality crystal and producing almostsimilar InAs quantum dots in the first, second and third layers. Theresults also reveal that the crystal can be produced without the misfittransfer even if the InGaAs layer having the thickness exceeding thecritical thickness calculated as shown in FIG. 8 is used, since thecomposition gradient is used. The surface density is 3.3×10¹¹ cm⁻² ofthe quantum dots, thereby achieving a high density level that isconventionally never provided. By increasing the composition gradientand optimizing the layered structure, a multi-layered structure havingfour or more layers can be provided.

When the Ga_(0.75)In_(0.25)As or Ga_(0.7)In_(0.3)As having the high Incontent is used on the InAs quantum dots, the quantum dots that emitlight at a wavelength of 1.3 μm or more, i.e., 1.308 μm or 1.325 μm canbe produced at a density of as high as 1.1×10¹¹ cm⁻². The half widthscan also be improved from 38.0 mV to 22.3 meV and 29.7 meV.

Production of High Density Quantum Dots

The density of the quantum dots are determined by the conditions upongrowth including the temperature, the speed and the pressure, which willbe described below.

Temperature Dependency

In Embodiment 1, when the growth temperature of the InAs layer isdecreased and the growth speed is increased, the diffusion length of anatom became short and the quantum dots having the high density could beobtained. The conditions are only for achieving the high density. If thegrowth temperature is increased, crystallinity becomes poor, and thecurrent applied does not contribute to the light emission and tendsescape as heat.

Next, the growth speed is fixed and the growth temperature is changed todetermine the light emission intensity properties.

FIG. 15 is a graph showing temperature dependency of light-emittingintensity of the semiconductor light-emitting element according to thepresent invention.

It shows the temperature dependency when the InAs quantum dots 5 of thesemiconductor light-emitting element and the InGaAs layer 6 are grown atthe growth speed of 0.1 ML/s.

It shows a relationship between the wavelength (abscissa: μm) when thegrowth temperature of the quantum dots is 550° C., 560° C., 570° C. or580° C. and the light-emitting intensity (ordinate: photons per unithour). Table 1 shows a sampling data of the temperature dependency. Inthe case of the growth temperature is 540° C., the result is alreadyshown and the data, therefore, is omitted.

The peak of the quantum dots grown at 550° C. is 143.5 at a frequency of1309 nm. The peak of the quantum dots grown at 560° C. is 400.2 at afrequency of 1321 nm. The peak of the quantum dots grown at 570° C. is980.6 at a frequency of 1311 nm. The peak of the quantum dots grown at580° C. is 302.0 at a frequency of 1303 nm.

The temperature dependency curve of the quantum dots grown at 550° C. isalmost not changed. The quantum dots grown at 570° C. had a steep peakin the temperature dependency curve. The temperature dependency curve ofthe quantum dots grown at 580° C. is again returned to the less changingstate. The peak of the temperature dependency curve has a trend to bechanged steeply shown in a doted line when the temperature is changed.The temperature dependency curve of the quantum dots grown at 570° C. ispreferable.

The quantum dots grown at 550° C. to 570° C. have steeply almostidentical size and the light emission intensity is increased. On theother hand, the quantum dots grown at 570° C. to 580° C. lost theirmaterials, especially indium by evaporation, and the numbers of thequantum dots are steeply decreased. Therefore, the light emissionintensity is steeply decreased. Even with the quantum dots grown at 580°C., the light emission intensity is in a usable frequency area.

As described above, the growth temperature of not less than 540° C. canbe applied.

Within the growth temperature of 540° C. to the evaporating temperatureof Indium, the light-emitting intensity can be increased. The growthtemperature is preferably 570° C.±100° C., and more preferably 570° C.TABLE 1 Temperature Wavelength (nm) 550° C. 560° C. 570° C. 580° C. 115012.0 14.4 15.2 1160 14.6 16.2 18.0 1170 17.2 22.2 25.4 1180 20.4 24.833.6 1190 25.8 26.4 66.2 1200 39.8 42.4 66.2 1210 45.6 51.2 81.6 122055.2 58.6 89.2 1230 68.6 73.0 99.2 1240 84.6 83.6 113.6 24.0 1250 97.099.8 131.0 34.0 1260 111.4 116.4 178.4 50.0 1270 122.0 137.2 228.0 76.01280 119.0 154.6 288.0 122.0 1290 125.4 180.6 435.4 208.0 1300 132.2225.0 723.8 298.0 1310 137.0 309.2 965.6 256.0 1320 137.8 397.6 850.2120.0 1330 132.2 319.4 430.6 36.0 1340 93.6 159.0 162.6 20.0 1350 43.259.8 63.2 10.0 1360 15.6 23.6 21.0 6.0 1370 9.8 14.2 14.6Increase in Light-Emitting Intensity by Providing High Density

FIG. 16 is a graph showing an increase in light-emitting emittingintensity by providing the high density of the sole semiconductorlight-emitting element according to the present invention. It shows arelationship between the wavelength (abscissa: nm) when the growthtemperature of the quantum dots is changed, and the light-emittingintensity (ordinate: photons per unit hour). Using the semiconductorlight-emitting element shown in FIG. 5, the measurement is made underthe conditions that the growth temperature is 570° C. and the growthspeed is 0.1 ML/s where ML/s represents Mono Layer/sec.

Table 2 shows a sampling data of the light-emitting intensity of thesole element.

When the surface density of the quantum dots is increased from 8×10⁹cm⁻² to 8×10¹⁰ cm⁻² to point up the one place with a decimal, the peakof the curve at a wavelength of 1288 μm and a light-emitting intensityof 250 obtained at the surface density of 8×10⁹ cm⁻² is changed to thepeak of the curve at a wavelength of 1319 μm and a light-emittingintensity of 2500 obtained at the surface density of 8×10¹⁰ cm⁻². Inother words, when the surface density is increased by 10 times, thelight-emitting intensity is also increased by 10 times. Consequently,the semiconductor light-emitting element according to the presentinvention has a tendency to increase the light-emitting intensitydepending on the high density of the quantum dots. TABLE 2 Quantum dotdensity Wavelength (nm) 8 × 10⁹ cm⁻² 8 × 10¹⁰ cm⁻² 1160 6 1180 20 401200 30 114 1220 10 172 1240 14 134 1260 76 136 1280 242 272 1300 1541154 1320 50 2432 1340 2 618 1360 76Relationship Between a Growth Rate and a Surface Density

FIG. 17 is a graph showing a relationship between a growth rate and asurface density of the quantum dots according to the present invention.

The measurement is made under the conditions that the growth temperatureis fixed to 570° C. and the growth speed is changed using the quantumdots having the structure shown in FIG. 5. Table 3 shows a sample dataof the relationship between the growth rate and the density.

It is the first time to produce the quantum dots using As₂ at the growthrate of not less than 0.006 ML/s according to the present invention.

The high density can be obtained by increasing the growth rate, i.e.,0.006 ML/s, 0.1 ML/s, 0.23 ML/s and 0.46 ML/s. According to the data, itcan be concluded that the surface density is proportion to the growthrate. It is preferable that the growth speed or the growth rate of thequantum dots be high. TABLE 3 Growth rate (ML/s) Surface density (cm⁻²)0.006 8.00E + 09 0.100 6.00E + 10 0.230 9.00E + 10 0.460 1.30E + 11Production of Strain Reducing Layer with Composition Gradient

In view of the above, in order to improve the strain reducing layer withthe composition gradient, the growth speed and the growth temperatureare preferably increased. In order to increase the growth speed, the useof the plurality of material feeding lines are effective. When the speedis further increased, the strain reducing layer with high quality can beproduced without the composition gradient.

InAs Quantum Dot Laser

The InAs quantum dot laser according to the present invention comprisesthe laser element having one layer of quantum dots as described above asa basic component. The plurality of the laser elements are layered toprovide the desired properties. The three-layered quantum dot laserelements will be described below.

FIG. 18 is a sectional view-of an InAs quantum dot laser comprisingthree-layered quantum dot laser elements according to the presentinvention.

1.5 μm of an Al_(0.75)Ga_(0.25)As layer having an n-type impuritydensity of 8×10¹⁷ cm⁻³ is laminated on an n-type GaAs substrate, and 210nm of the three-layered quantum dot laser element that became awaveguide are laminated thereon (see FIG. 7).

1.5 μm of an Al_(0.75)Ga_(0.25)As layer having a p-type impurity densityof 7×10¹⁷ cm⁻³ is laminated on the three-layered quantum dot laserelement. On the p-type layer, a p-type contact is formed. An electrodeof AuGe(100 nm)/Au(100 nm) is disposed on the n-GaAs substrate. Anelectrode of Ti(30 nm)/Pt(10 nm)/Au(150 nm) is disposed on thep-contact. The GaAs layer 2 (3) between the InAs thin film layer 4 andthe InGaAs layer 6 had a thickness of 26 nm. In this case, a lightconfinement coefficient is 0.0624, and the quantum dot density is 8×10¹⁰cm⁻².

Laser Characteristics 1

FIG. 19 shows characteristics of the semiconductor laser elementobtained by the following conditions. Table 4 shows a sampling data.

Conditions

Using five layered high density quantum dots

Base level oscillation with short resonator length and without HR mirror(1.316 μm)

R.T., Pulse, room temperature (high frequency pulse)

Resonator length L=0.89 mm

CL/CL (only cleavage plane, no HR coat)

Laser is oscillated at 500 mA or more. The laser can oscillate with astructure having only cleavage plane and with a laser oscillation lengthof 0.89 mm. This is because the number of the quantum dots is great,whereby many carriers are treated. TABLE 4 Current (mA) Power [mW]Voltage [V] 0 0 0.000000 50 0 1.323703 100 0 1.616985 150 2.57B−031.835083 200 4.68E−04 1.985856 250 4.79E−03 2.176944 300 6.66E−032.316026 350 6.78E−03 2.449666 400 1.23E−02 2.606084 450 1.52E−022.748996 500 9.36E−02 2.854820 540 1.61E+00 2.974149Laser Properties 2

FIG. 20 shows I-V (current-voltage) characteristics and I-L(current-light quantity) characteristics of the semiconductor laserelement obtained by the following conditions. Table 5 shows a samplingdata.

Conditions

Using the three layered structure

Ld action without HR coat (high reflectance mirror

R.T., Pulse, room temperature (high frequency pulse)

Reasonator length L=3 mm

CL/CL (only cleavage plane, no HR coat)

Laser is oscillated at 1000 mA or more. The laser can oscillate with astructure having only cleavage plane and with a laser oscillation lengthof 3 mm. This is because the number of the quantum dots is great,whereby many carriers are treated. TABLE 5 Current (mA) Output Power[mW] Voltage [V] 0 0 0 100 5.13E−03 1.452304 200 1.34E−02 1.756471 3002.93E−02 1.997144 400 3.94E−02 2.183394 500 4.80E−02 2.352510 6005.90E−02 2.484336 700 7.87E−02 2.632086 800 8.93E−02 2.725211 9001.06E−01 2.793543 1000 1.63E−01 2.889087 1025 9.73E−01 2.914887

Various modification and alternations of the structure of the elementand the method of producing the same that do not depart from the scopeand intent of the present invention will become apparent to thoseskilled in the art.

According to the present invention, the following advantages can beprovided.

(1) The arsenic material is changed from AS₄ to As₂, whereby theproduction method that cannot be used for As₄ can be used.

(2) The growth temperature and the growth speed can be optimized. Inparticular, the second InAs thin film layer having the plurality of InAsquantum dots is produced at the growth temperature of not less than 540°C. and at the growth speed of not less than 0.006 ML/s, whereby thequantum dot density can be improved and the light-emitting intensity canbe increased.

(3) InGaAs having the high In content is used, whereby the latticeconstants can be matched.

(4) The InGaAs layer with a modified composition or composition gradientis used to increase the density and quality of the quantum dots thatemit light at a wavelength of 1.3 μm.

(5) The planar semiconductor light-emitting element is used to adjustthe area for handling light, and to increase the number of quantum dots.

1. A semiconductor light-emitting element, comprising: a first GaAslayer, a second InAs thin film layer having the plurality of InAsquantum dots formed on the first GaAs layer, a third InGaAs layer formedon the second InAs thin film layer having the plurality of InAs quantumdots, and a fourth GaAs layer formed on the third InGaAs layer, whereinthe As source is As₂.
 2. A semiconductor light-emitting elementaccording to claim 1, wherein the second InAs thin film layer having theplurality of InAs quantum dots formed on the first GaAs layer is formedat a growth temperature of not less than 540° C. and at a growth speedof not less than 0.006 ML/s.
 3. A semiconductor light-emitting elementaccording to claim 1 or 2, wherein the light-emitting element has alight emission wavelength within a range of 1.28 to 1.34 mm, a surfacedensity of not less than 6×10¹⁰ cm⁻² and an emission half width of notmore than 40 meV.
 4. A method of producing a semiconductorlight-emitting element, comprising the steps of: forming a GaAs layer ona semiconductor substrate, forming an InAs thin film layer having theplurality of InAs quantum dots on the GaAs layer, forming an InGaAslayer on the InAs thin film layer having the plurality of InAs quantumdots, and forming another GaAs layer on the InGaAs layer, wherein Assource is As₂.
 5. A method of producing a semiconductor light-emittingelement according to claim 4, wherein the InAs thin film layer havingthe plurality of InAs quantum dots is produced at a growth temperatureof not less than 540° C.
 6. A method of producing a semiconductorlight-emitting element according to claim 5, wherein the InAs thin filmlayer having the plurality of InAs quantum dots is produced at a growthtemperature of not less than 540° C. but not more than the evaporatingtemperature of indium.
 7. A method of producing a semiconductorlight-emitting element according to any one of claims 4 to 6, whereinthe InAs thin film layer having the plurality of InAs quantum dots isproduced at a growth speed of not less than 0.006 ML/s.
 8. A method ofproducing a semiconductor light-emitting element according to claim 4,wherein the InAs thin film layer having the plurality of InAs quantumdots is produced at a growth temperature of not less than 540° C. and ata growth speed of not less than 0.006 ML/s.
 9. A laminated semiconductorlight-emitting element comprising the plurality of the semiconductorlight-emitting elements according to claim 1 vertically stacked on oneanother, wherein the first layer of one semiconductor element islaminated on the fourth layer of another semiconductor element.
 10. Alaminated semiconductor light-emitting element according to claim 9,wherein an InGaAs layer having a high In content is formed at aninterface between the InAs quantum dots and the InGaAs layer, andwherein the amount of In contained in the InGaAs layer is graduallydecreased in a direction away from the interface.
 11. A method ofproducing a semiconductor light-emitting element, comprising the stepsof: forming a GaAs layer on a semiconductor substrate, forming an InAsthin film layer having the plurality of InAs quantum dots on the GaAslayer, forming an InGaAs layer on the InAs thin film layer having theplurality of InAs quantum dots, forming another GaAs layer on the InGaAslayer, and repeating the steps so that the desired number of thesemiconductor light-emitting elements are vertically stacked, wherein Assource is As₂.
 12. A method of producing a semiconductor light-emittingelement according to claim 11, wherein an InGaAs layer having a high Incontent is formed at an interface between the InAs quantum dots and theInGaAs layer, and wherein the amount of In contained in the InGaAs layeris gradually decreased in a direction away from the interface emittingelement according to claim 11 or 12, wherein the
 13. A method ofproducing a semiconductor light-element according to claim 11 or 12,wherein the InAs thin film layer having the plurality of InAs quantumdots is produced at a growth temperature of not less than 540° C.
 14. Amethod of producing a semiconductor light-emitting element according toclaim 13, wherein the InAs thin film layer having the plurality of InAsquantum dots is produced at a growth temperature of not less than 540°C. but not more than the evaporating temperature of indium.
 15. A methodof producing a semiconductor light-emitting element according to claim11 or 12, wherein the InAs thin film layer having the plurality of InAsquantum dots is produced at a growth speed of not less than 0.006 ML/s.16. A method of producing a semiconductor light-emitting elementaccording to claim 11 or 12, wherein the InAs thin film layer having theplurality of InAs quantum dots is produced at a growth temperature ofnot less than 540° C. and at a growth speed of not less than 0.006 ML/s.17. A semiconductor light-emitting element according to claim 1, 2, 9 or10, wherein the semiconductor light-emitting element is a planarsemiconductor light-emitting element.